1. Field of the Invention
This invention relates to a radio communication device and a semiconductor integrated circuit used for the same. More specifically, the present invention relates to a radio communication device employing a channel quality estimation method, a method for deciding a channel to be used and a method for deciding transmission power in a radio data communication of a frequency hopping system and relates to a semiconductor integrated circuit device used for the same.
2. Description of the Related Art
In recent years, a new radio communication system to connect electronic appliances with one another by radio has been developed. The IEEE 802.11 b/g, Bluetooth (trade mark) and the like are known as this kind of radio communication system.
The IEEE 802.11 b/g is a standard of a short-range radio communication system proposed for a wireless local area network (LAN). The Bluetooth is a standard of a short-range radio communication system proposed for connection among not only computers but a variety of appliances with one another. In these radio communication systems, the 2.4 GHz band called an industrial scientific and medical band (ISM band) is used as a frequency band allowed to be freely used without any license on a condition that a prescribed standard is satisfied. The IEEE 802.11 b/g adopts a direct sequence spread spectrum (DSSS) technology so as to maximize influences of noise signals generated from other electronic appliances, for example, electronic ovens or the like present in the ISM band of the 2.4 GHz band. The Bluetooth adopts a frequency hopping spread spectrum (FHSS) technology of the frequency hopping system. Each technology, then, achieves sufficient noise-resistance for the radio communication system.
More specifically, a Bluetooth-compatible portable device (hereinafter, referred to as Bluetooth device) is used to exchange data among a cellular phone, a personal digital assistant (PDA), a notebook personal computer, a sound terminal appliance, etc. In this case, the frequency hopping system is adopted, wherein one channel is selected from among 79 frequency channels defined in a frequency band from the 2.40 GHz to the 2.48 GHZ and it is switched with the lapse of time to make a radio communication. This frequency hopping system repeatedly selects channels at every fixed time period, for example, at every 625 μs on the basis of a preset pseudo random algorithm and assigns one packet of data to the one channel to make a communication. On the other hand, a wireless LAN station using the 2.4 GHz band does not employ frequency hopping but employs a constantly fixed and set frequency band, namely, contiguous frequency widths equivalent to almost 20 channels of the Bluetooth device.
FIG. 1 shows frequency relationships when the Bluetooth device and the wireless appliance use the ISM band of the 2.4 GHz. The Bluetooth divides almost entire areas of the ISM band of the 2.4 GHz into 79 channels of 1 MHz width and communicates while sequentially changing the frequencies used by the 79 channels at every 625 μs in accordance with a preset sequence defined by appliance addresses or the like.
On the other hand, the wireless LAN defines total 13 channels in the ISM band of the 2.4 GHz. A band width occupied by the one channel is 20 MHz and these 13 channels are arranged in a manner that a part of them are overlapped one another, as shown in FIG. 1. Any one channel is assigned to each access point of the wireless LAN at the time of setting thereof, and communications are made by suing the assigned channels. The influences of the noise signals are reduced by employing the DSSS system. Channels are assigned to each access point so as not interfere with one another when a plurality of access points are arranged in order to overlap mutual service areas.
As shown in FIG. 1, presence of the Bluetooth device and the wireless LAN appliance in an identical frequency area causes a mutual communication to be interfered by radio waves transmitted from each other. To meet this interference, the Bluetooth device adopts an adaptive frequency hopping (AFH) technology. The AFH technology prevents the interference between the Bluetooth device and wireless LAN appliance in a manner that the Bluetooth device observes the channels in any method to avoid the channel to be determined the presence of a radio wave to interfere its own communication and performs the frequency hopping.
To achieve the AFH, it is needed for the Bluetooth device to observe states of each channel and determine which channel should be used. Therefore, it is expected to adopt a method for estimating channel quality and method for deciding a channel to be used which decides the frequency to be used so as to avoid, with an appropriate response speed, use of channels with poor qualities by detecting channels surely influenced from others, namely channels with bad in quality. The Bluetooth device is frequently adapted to a small sized appliance with a small battery capacity such as a cellular phone and a head set needed to minimize its consumption current. The Bluetooth device also needs to avoid increase, as much as possible, in the consumption current resulting from the adaptation of the method for estimating the channel quality and the method for deciding the channel to be used.
In general, a method for a certain radio appliance to estimate a quality of channel to be used by itself includes a method for measuring a field strength of a radio wave present in the channel to be used prior to a communication (Passive method) and a method for assuming that the quality of the channel is poor when error rates of user data and control data in communication exceed preset values (Active method).
The former Passive method directly measures the filed strength, so that the Passive method can quickly detect interference from other appliances. However, since the radio appliance has to perform reception operations other than receptions of the user data and the control data, the consumption current is increased. A variety of error correction technologies are introduced into packets of the Bluetooth and if the interference is minor, the technologies can be suppressed affections of the interference by means of those correction technologies. But, in the case of the Passive method, it is impossible to know the degree of the actual affections of the measured radio waves affected on communications.
On the contrary, since the latter Active method detects the influence from other radio appliances by measuring the error rates of the user data and control data, the Active method has some advantages. That is, it is determined whether the influence is serious or not by adding actual affections of radio waves from interference sources on the communications, and the consumption current is not increased because the radio appliance does not receive other than the user data and control data. However, the Active method has problems such that a plurality of items of data has to be received to measure the error rates and it takes a long time to determine each channel condition. And the Active method further has a problem that interference waves are varied in strength with the elapse of time. The variations due to the elapse of time are caused because a radio wave propagation environment is varied with the elapse of time (fading phenomenon) or the use of channels is originally bursty like the wireless LAN. Since the strengths of the interference waves vary as the time goes on because of those causes, this variations of affections by the interference are also generated as a problem.
Furthermore, on the case of avoidance of use of channels with very poor qualities by means of the AFH, when qualities of most of channels in the ISM band become poor, it is worried that radio waves to be transmitted from the Bluetooth are collected to a part of the ISB band to influence adverse effects on other appliances.
The fact that it is possible for an evaluation of a channel quality of a radio communication using the AFH to use a bit error rate, a packet error rate, a signal to noise ratio (S/N) or the like is disclosed in Japanese Patent No. 3,443,094.